The following paragraphs are not an admission that anything discussed in them is prior art or part of the knowledge of persons skilled in the art.
Fuel cells have been proposed as a clean, efficient and environmentally friendly source of power that can be utilized for various applications. A fuel cell is an electrochemical device that produces an electromotive force by bringing the fuel (typically hydrogen) and an oxidant (typically air) into contact with two suitable electrodes and an electrolyte. A fuel, such as hydrogen gas, for example, is introduced at a first electrode (anode) where it reacts electrochemically in the presence of the electrolyte to produce electrons and cations. The electrons are conducted from the anode to a second electrode (cathode) through an electrical circuit connected between the electrodes. Cations pass through the electrolyte to the cathode. Simultaneously, an oxidant, such as oxygen gas or air is introduced to the cathode where the oxidant reacts electrochemically in presence of the electrolyte and catalyst, producing anions and consuming the electrons circulated through the electrical circuit; the cations are consumed at the cathode. The anions formed at the cathode react with the cations to form a reaction product. The anode may alternatively be referred to as a fuel or oxidizing electrode and the cathode may alternatively be referred to as an oxidant or reducing electrode. The half-cell reactions at the two electrodes are, respectively, as follows:H2→2H++2e−  (1)½O2+2H++2e−→H2O  (2)
The external electrical circuit withdraws electrical current and thus receives electrical power from the fuel cell. The overall fuel cell reaction produces electrical energy as shown by the sum of the separate half-cell reactions written above. Water and heat are typical by-products of the reaction. Accordingly, the use of fuel cells in power generation offers potential environmental benefits compared with power generation from combustion of fossil fuels or by nuclear activity. Some examples of applications are distributed residential power generation and automotive power systems to reduce emission levels.
In practice, fuel cells are not operated as single units. Rather, fuel cells are connected in series, stacked one on top of the other, or placed side by side. A series of fuel cells, referred to as fuel cell stack, is normally enclosed in a housing. The fuel and oxidant are directed through manifolds to the electrodes, while cooling is provided either by the reactants or by a separate cooling medium. Also within the stack are current collectors, cell-to-cell seals and insulation. Piping and various instruments are externally connected to the fuel cell stack for supplying and controlling the fluid streams in the system. The stack, housing, and associated hardware make up the fuel cell unit.
There are various known types of fuel cells. For example, proton exchange membrane (PEM) fuel cells are one of the most promising replacements for traditional power generation systems, as a PEM fuel cell enables a simple, compact fuel cell to be designed, which is robust and which can be operated at temperatures not too different from ambient temperatures. Usually, PEM fuel cells are fuelled by pure hydrogen gas, as it is electrochemically reactive and the by-products of the reaction are water and heat, which is environmentally friendly. A conventional PEM fuel cell usually comprises two flow filed plates (bipolar plates), namely, an anode flow field plate and a cathode flow field plate, with a proton exchange membrane (MEA) disposed there between. The MEA includes the actual proton exchange membrane and layers of catalyst for fuel cell reaction coated onto the membrane. Additionally, a gas diffusion media (GDM) or gas diffusion layer (GDL) is provided between each flow field plate and the PEM. The GDM or GDL facilitates the diffusion of the reactant gas, either the fuel or oxidant, to the catalyst surface of the MEA while provides electrical conductivity between each flow field plate and the PEM.
Each flow field plate typically has three apertures or openings at each end, each aperture representing either an inlet or outlet for one of fuel, oxidant and coolant. However, it is possible to have multiple inlets and outlets on flow field plates for each reactant gas or coolant, depending on the fuel cell or stack design. When a fuel cell stacked in assembled, the anode flow field plate of one cell abuts against the cathode flow field plate of an adjacent cell. These apertures extend throughout the thickness of the flow field plates and align to form elongate distribution channels extending perpendicular to the flow field plates and through the entire fuel cell stack when the flow field plates stack together to form a complete fuel cell stack. A flow field usually comprises at least one, and in most cases a plurality of, open-faced flow channels that fluidly communicate (connect) appropriate inlet and outlet. As a reactant gas flows through the channels, it diffuses through GDM and reacts on the MEA in the presence of catalyst. A continuous flow through ensures that, while most of the fuel or oxidant is consumed, any contaminant are continually flushed through the fuel cell. The flow field may be provided on either face or both faces of the flow field plate. Typically, fuel or oxidant flow fields are formed respectively on the face of the anode and cathode flow field plate that faces toward the MEA (hereinafter, referred to as “front face”). A coolant flow field may be provided on either the face of either of anode or cathode flow field plate that faces away from the MEA (hereinafter, referred to as “rear face”).
When a complete fuel cell stack is formed, a pair of current collector plates (bus bars) are provided immediately adjacent the outmost flow field plates (starter plates), one on each side of the stack, to collect current from the fuel cell stack and supply the current to an external electrical circuit. A pair of insulator plates is provided immediately outside of the current collector plates followed by a pair of end plates located immediately outside the insulator. Alternatively, an end plate may be utilized, which has an electrically insulating coating on the outer surface or the endplate may be manufactured using an electrically insulating material. A seal is provided between each pair of adjacent plates. The seal is usually in the form of gaskets made of resilient materials that are compatible with the fuel cell environment. A fuel cell stack, after assembly, is commonly clamped to secure the elements and ensure that adequate compression is applied to the seals and active areas of the fuel cell stack. This method ensures that the contact resistance is minimized and the electrical resistance of the cells is at a minimum.
For the purposes of this patent application, the term “insulator end plate” is used to describe either a first alternative having a combination of an insulator plate and end plate, or a second alternative having an end plate manufactured with electrically insulating material or coated on the outer surface with an electrically insulating layer.
The bus bar is arranged in a recess or pocket provided in the insulator plate, for the first alternative of an insulator end plate, or in the end plate itself, for the second alternative.
The depth of the recess is ideally slightly smaller than the thickness of the bus bar, so that when the bus bar is placed in the recess, the bus bar protrudes a certain distance from the flat side of the insulator plate/end plate to ensure good contact between bus bar and starter plate (first flow field plate of the stack). If the recess is deeper than the thickness of the bus bar, the bus bar will not be pressed against the starter plate and, thus, the electrical contact between the bus bar and the starter plate will be poor. In this situation, an elastic member may be inserted between the recess and the bus bar, to press the bus bar towards the starter plate.
It can be appreciated from the previous discussion that a problem in conventional fuel cell is that the amount of bus bar protruding from the recess cannot be too great. If the bus bar protrudes too much, the adjacent flow field plates may crack from the shear stresses created when the stack is compressed.